Large Evanescent Field Research Articles

Electric-field sensor based on propylene carbonate cladding microfiber sagnac loop interferometer

Optical microfiber with diameter of wavelength scale has gained numerous successful applications in many fields, as it exhibits a number of unique characteristics and excellent optical properties such as strong confinement, physical flexibility, large evanescent fields, extreme configurability, and high robustness. In this letter, we propose a compact microfiber-based electric field sensor, which consists of a single-mode microfiber sagnac loop interferometer (SMSLI) and propylene carbonate. Owing to Kerr electro-optic effect of propylene carbonate, the refractive index (RI) of propylene carbonate cladding in the twist area of SMSLI will be changed by the applied electric field, which induces a shift of the interference pattern of SMSLI. Hence, the external electric field can be demodulated by measuring the variation of the transmission spectra of SMSLI. The feasibility of the proposed sensor is verified by detecting alternating electric field with frequency of 50 Hz and impulse electric field with duration time of 200 μs.

Measurement of refractive index and temperature using a balloon-like interferometer with an inner embedded waist-enlarged structure

A balloon-like composite optical fiber interferometer, integrated with a waist-enlarged structure, is presented for measuring refractive index (RI) and temperature. The waist-enlarged fusion tapers (WEFTs) are spliced to the single-mode fiber (SMF) to form a Mach-Zehnder interferometer (MZI). Meanwhile, the WEFTs structure is curved into a balloon-like structure. When the two structures are integrated, a larger evanescent field is formed, which can interact with the external environment better. The experiment results indicate that the best sensitivity of RI and temperature can achieve 340.9 nm/RIU and −0.162 nm/℃, respectively. In addition, the interferometer is small, easy to manufacture, and can be widely applied in various measurement applications.

Particle trapping with optical nanofibers: a review [Invited

Optical trapping has proven to be an efficient method to control particles, including biological cells, single biological macromolecules, colloidal microparticles, and nanoparticles. Multiple types of particles have been successfully trapped, leading to various applications of optical tweezers ranging from biomedical through physics to material sciences. However, precise manipulation of particles with complex composition or of sizes down to nanometer-scales can be difficult with conventional optical tweezers, and an alternative manipulation tool is desirable. Optical nanofibers, that is, fibers with a waist diameter smaller than the propagating wavelength of light, are ideal candidates for optical manipulation due to their large evanescent field that extends beyond the fiber surface. They have the added advantages of being easily connected to a fibered experimental setup, being simple to fabricate, and providing strong electric field confinement and intense magnitude of evanescent fields at the nanofiber's surface. Many different particles have been trapped, rotated, transported, and assembled with such a system. This article reviews particle trapping using optical nanofibers and highlights some challenges and future potentials of this developing topic.

Design of a high-sensitivity polymer double-slot waveguide sensor for point-of-care biomedical applications

Silicon photonics is a rapidly developing field that offers cost-effective biosensors with improved sensitivity and the potential for interconnecting with other electronic devices for instant disease diagnosis. The Mach Zehnder interferometer architecture (MZI) is a key technology for biosensors, as it detects changes in refractive index (RI) caused by the presence of biomolecules. In this study, a silicon-polymer double-slot waveguide-based MZI was designed, with a small mode area and a large evanescent field to enhance light-analyte interaction. The waveguide was optimized by converting a normal slot waveguide into a double-slot waveguide with varying slot widths. In transmission spectrum, the wavelength shift was measured for both normal and disease samples. Additionally, the loss at a specific wavelength was analyzed to understand the impact of the biomolecule on the sensor performance. The results show that this sensor has a high sensitivity of 2.39 X10^5 nm/RIU, making it a promising candidate for biosensing applications.

Batch fabrication of the smooth-surfaced and tailored-diameter microfiber Bragg grating sensors using hydrothermal method and its characteristics

Microfiber Bragg grating sensors hold great promise given their compactness and large evanescent field effect. The fabrication method of the sensors plays a pivotal role in determining the performance. In this study, a novel approach for batch manufacturing microfiber Bragg grating sensors was proposed using a NaOH solution under hydrothermal conditions. Compared to other methods, it is simpler, safer, and more efficient. Through precisely controlling the reaction time, 50 microfiber Bragg grating sensors were produced with smooth surfaces, exhibiting a surface roughness of about 87 nm, and achieved tailored fiber diameters with an error of only ± 1.2918 μm. In addition, the sensors with different diameters produced by the method to systematically investigate the effects of diameter reduction on various aspects, including spectral characteristics, temperature, strain, refractive index sensing, and bending performance, accompanied by theoretical explanations.

A Compact, Highly Sensitive pH Sensor Based on Polymer Waveguide Bragg Grating

Accurate probing, monitoring, and controlling the pH level is critical in different fields. In the last two decades, optical fiber/waveguide-based pH sensors have been widely investigated with the aim of further enhancement of sensitivity and miniaturization. Here, we propose a novel and cost-effective pH sensor based on polymer waveguide Bragg grating, addressing main aspects like sensitivity enhancement, miniaturization, narrow band response, and stable performance. The large evanescent field due to metal under cladding and high index coating combined with excellent penetration depth match with pH-sensitive polymer coating of poly-acrylic acid/poly-allylamine hydrochloride (PAA/PAH) in an aqueous medium, ensures a sensitivity of 10.4 nm/pH for a pH range of 4 to 7, two orders higher than all reported Bragg grating pH sensors to our knowledge. A narrow band (∼1 nm) response has been ensured by low propagation loss of the TE mode. Further, the variations in sensitivity and peak reflectivity are within 3% for a ±10% inaccuracy in various structural parameters, reflecting excellent fabrication tolerances. The proposed sensor is an important development, especially for the perishable food industry in which a high sensitivity in a pH range of 4 to 7 is highly desirable to accurately determine the freshness of foods.

Optical Nanofiber Skins for Multifunctional Humanoid Tactility

Humanoid tactility has been boosting robotic intelligence in object recognition, dexterous manipulation, and human–robot interaction. For many artificial tactile sensors, especially those based on optical principles, inflexibility, bulkiness, and monomodality limit their potential to function as humanoid skins. Herein, by embedding lab‐made optical nanofibers (ONFs) into elastomeric films, soft, flexible, thin (around 500 μm, similar to human skin), and multimodal (force and thermosensitive) robotic skins are achieved. These superior characteristics arise from the low flexural rigidity and large evanescent field of ONFs, due to their subwavelength diameters (down to 450 nm). By tuning light wavelength, ONF diameter, and skin thickness, variable sensitivities and sensing ranges for both force and temperature are reported. Depending on different sensing requirements, special modules can be further assembled on the ONF skins for various surface properties, including hardness, texture, and thermal conductivity. The ONF skins can enable a commercial robot to emulate human behaviors, including adaptive grasping of flimsy objects, contactless temperature measurement, and even the perception of leaf veins. It is anticipated that these ONF skins could offer a unique solution in multiple intelligent systems such as robotics, prosthetics, human–machine interface and wearable devices.

Highly sensitive microfiber ultrasound sensor for photoacoustic imaging

An ultrasound wave is a kind of acoustic signal with a frequency greater than 20 kHz, which is widely used in diverse fields such as medical imaging diagnosis, nondestructive testing and resource exploration. A variety of ultrasound sensors have been developed for ultrasound detection. Particularly for photoacoustic imaging, specialized ultrasound sensors with high sensitivity, small size, and broad bandwidth are needed. However, achieving such sensor performance still poses a great challenge to the current state-of-the-art in ultrasound sensor technology. A recent work published in <italic>Opto-Electronic Advances</italic> (DOI: 10.29026/oea.2022.200076) proposes a microfiber-based ultrasound sensor that breaks the limitations of existing ultrasound sensors. Benefiting from the large evanescent field characteristic of microfiber, combined with the coherent detection technology, the proposed sensor realized highly sensitive ultrasound detection and demonstrated excellent performance in high-resolution photoacoustic imaging. The highly sensitive and miniaturized microfiber ultrasound sensor provides a competitive alternative for various applications, such as endoscopic photoacoustic imaging of the intestinal tract and blood vessels in animals.

Thermo-optically tunable slot waveguide-based dual mode-splitting resonators with enhanced sharp lineshapes.

Slot waveguide plays an essential role in achieving high-performance on-chip photonic sensors and nonlinear devices. Ideally, slot waveguide features a large evanescent field ratio and strong electric field intensity in the slot, leading to a high waveguide sensitivity. Unfortunately, the microring resonator (MRR) based on the slot waveguide suffers the less steep spectral slope due to the low quality factor induced by the huge optical propagation loss of the slot waveguide. In this work, a novel dual mode-splitting resonator based on the slot waveguide is proposed and demonstrated to steepen the slope of lineshapes. The device is implemented by two racetrack resonators based on a slot waveguide and a feedback waveguide to introduce coherent optical mode interference, which could induce mode-splitting resonance (MR) with sharp asymmetry line shape and large extinction ratio (ER). The proposed device is fabricated by the standard complementary metal-oxide-semiconductor (CMOS) technologies on silicon-on-insulator (SOI) platform, and the characterization results show dual MRs with an ER of 45.0 dB and a slope rate (SR) of 58.3 dB/nm, exhibiting a much steeper lineshape than that of the conventional MRR with slot waveguide. And the resonance can be tuned efficiently by applying various voltages of the TiN microheater. Investigations in dual MRs devices promote many potential applications in the field of optical switching, optical modulating, and on-chip optical sensing.

Bragg gratings and BIO-Bragg-gratings in tapered optical fibers

Tapered optical fibres are well-established devices for different applications, in order to exploit nonlinear effects, manage dispersion, excite azimuthal resonances in microresonators (so-called Whispering Gallery Modes). Also, the micro- or nanofibres guide optical-fields with large evanescent fields. In this talk, we will focus on the combination of tapers and Bragg gratings to perform novel optical devices. We will present two applications: the first, the fabrication of single-mode Bragg gratings in commercial multimode or few-mode tapered fibres by means of UV-photoinscription. The result is a grating that exhibits a single-mode reflection band and low insertion loss. The second application is the fabrication of Bio Bragg Gratings in micrometric tapers of single-mode fibres. In this case, the Bragg Grating is formed by a periodic pattern of biomolecules microstamped on the surface of the taper waist. As the molecules binds to its specific antibody, the reflectivity of this Bio Bragg Grating will increase, allowing quantification of the antibody concentration.

Highly sensitive and miniature microfiber-based ultrasound sensor for photoacoustic tomography

A microfiber with large evanescent field encapsulated in PDMS is proposed and demonstrated for ultrasound sensing. The compact size and large evanescent field of microfiber provide an excellent platform for the interaction between optical signal and ultrasound wave, exhibiting a high sensitivity of 3.5 mV/kPa, which is approximately 10 times higher than the single-mode fiber sensor. Meanwhile, a phase feedback stabilization module is introduced into the coherent demodulation system for long-term stable measurement. In addition, a photoacoustic tomography experiment with the microfiber ultrasound sensor is implemented to verify the excellent performance on imaging, with the depth of 12 mm, the highest lateral resolution of 65 μm and axial resolution of 250 μm, respectively. The highly sensitive microfiber ultrasound sensor provides a competitive alternative for various applications, such as industrial non-destructive testing, biomedical ultrasound and photoacoustic imaging.

Sub-femtogram sensing by engineering evanescent field on a single serially grafted waveguide

The intensity of the evanescent field (EF) is vital for the sensitivity and limit of detection (LoD) of optical fiber or waveguide EF sensors. The fusion of ultra-highly sensitivity, simplicity, and cost effectiveness is still a major research field of sensing system despite the advances achieved during the last decades. In this study, we engineered EF by coating a high-refractive-index layer on a polymer waveguide that was serially grafted onto optical fibers. The simulation results show that the proportion of EF intensity can be significantly increased by 2–3 orders of magnitude compared with that of the non-coated waveguide. As a proof-of-concept, we fabricated Ta2O5 coated optical fiber-waveguide-fiber (OFWF) sensors via laser-induced waveguide self-writing and electron beam deposition techniques. The absorbance- and fluorescence-based sensing properties of the coated OFWF were investigated by spectral measurement of rhodamine B aqueous solutions. Owing to large EF ratio and high adsorption capacity of the sensor, we achieved a spectral sensing LoD of 9.5 × 10−16 g/ml with absolute mass of 0.4–0.5 fg.

Ultrahigh Sensitivity Mach-Zehnder Interferometer Sensor Based on a Weak One-Dimensional Field Confinement Silica Waveguide.

We report a novel Mach−Zehnder interferometer (MZI) sensor that utilizes a weak one-dimensional field confinement silica waveguide (WCSW). The WCSW has a large horizontal and vertical aspect ratio and low refractive index difference, which features easy preparation and a large evanescent field for achieving high waveguide sensitivity. We experimentally achieved WCSW ultrahigh waveguide sensitivity of 0.94, MZI sensitivity of 44,364 π/RIU and a low limit of detection (LOD) of 6.1 × 10−7 RIU.

Identification of Recycled Cooking Oil by Varying Number of Au/GO Coated Fiber Optic’s Loop Ring

This study investigates the effect gold nanoparticles (Au)/graphene oxide (GO) coated fibre optics with various number of loop ring in identifying recycled cooking oil based on their frying time. Various layers of gold nanoparticles (AuNP) suspension were deposited onto the substrate via drop casting technique. Few material characterizations were performed such as Fourier Transform Infrared (FTIR) spectroscopy, Ultraviolet Visible (UV-Vis) spectroscopy and refractometer analysis. Three macrobending structures of Au/GO coated fiber optic’s loop ring such as single, double and triple loops with diameter of 1.5cm were formed to generate large area of evanescent field around the fibre and to excite surface plasmon polaritons due to the interaction between AuNPs and analytes. Cooking oil samples with different frying time such as 14 minutes, 28 minutes and 42 minutes were prepared as analytes. The FTIR analysis revealed some changes in the transmittance percent of some bands as well as some slight shifts in the exact position of the bands due to the different of fatty acid composition. As the Au/GO coated fibre optics were immersed into the samples, the penetration depths were obtained as 142.75nm and 168.91nm at operating wavelengths of λ=1310nm and λ=1550nm, respectively. By using 1550nm operating wavelength due to its greater penetration depth than 1310nm, triple loop rings fibre optics coated with two layers of AuNP and one layer of GO exhibits a linear consistency with maximum sensitivity up to 69.17% due to the formation of large evanescent field around the loop area and intense SPP excitation.

All‐fibre phase shifter based on tapered fibre coated with MoWS 2 ‐rGO

In this work, an all-fibre phase shifter using tapered fibre coated with molybdenum tungsten disulfide - reduced graphene oxide (MoWS2-rGO) is demonstrated. The large evanescent field of tapered fibre allows MoWS2-rGO to absorb injected light from a 1400-nm pump. It subsequently modifies the refractive index of both an MoWS2-rGO solution and tapered fibre because of the thermo-optic effect. A maximum wavelength shift of 2.489 nm is observed and the equivalent phase shift is 0.91π. The efficiency of the phase shifter is calculated at 0.0175 nm/mW. The proposed phase shifter is beneficial for a variety of applications, specifically for future optical beam-forming networks and all optical signal processing devices.

Reconfigurable THz Metamaterial Filter Based on Binary Response for Information Processing System

Light-matter interactions between the metallic and dielectric layers along with the controlling of electromagnetic waves can create a way to develop micro-devices and moderate the functionalities for advanced applications. This study describes a new controlling technique of the plasmatic electron packet based on an electric split-ring resonator (eSRR). All numerical experiments were performed using an advanced CST electromagnetic package. The proposed metamaterial tunneled structure in this study operates using terahertz (THz) frequency spectrum as an efficient digital processing filter. The array combination of the tunneled structure consisted of three individual unit cells. Moreover, the two engineered metallic arms added to the tunneled structure exhibited two peak resonances and one passband frequency region. A large evanescent field was produced to enhance the wave-metal interactions with the presence of a metal-dielectric micro-tunnel. The intensity of the electromagnetic wave-metal interactions was encoded to binary 0 and 1 for information encoding purposes. As a result, the reconfigurable micro-unit cell metamaterial tunneled structure was able to effectively control the electric field and allow electron packets to be digitally encoded for the information processing system.

All fiber temperature sensor based on light polarization measurement utilizing graphene coated tapered fiber

AbstractIn this work, all fiber temperature measurement based on change of light polarization is demonstrated using tapered fiber coated with graphene. The large evanescent field of tapered fiber allows graphene to absorb the surrounding heat and subsequently this changes the refractive index of graphene coated tapered fiber. The change of refractive index is due to thermo‐optic effect. When there is a change in surrounding temperature, refractive index of the coated tapered fiber changes and this subsequently modifies the injected light polarization. The proposed temperature sensor can measure temperature up to 250°C and its sensitivity is ascertained at 0.48°/°C. The proposed temperature sensor is beneficial for future Internet of Things (IoT) applications.

Modal Characteristics of Refractive Index Engineered Hybrid Plasmonic Waveguide

In this work, we proposed a novel design of a refractive index engineered subwavelength grating silicon-based hybrid plasmonic waveguide via the finite element method. The intensity integral ( $\Gamma$ ) in the upper cladding can be controlled by varying the thickness ( ${H}_{\textit {Si}}$ ) of the top silicon layer. The highest evanescent field ratio of 0.85 is obtained at ${H}_{\textit {Si}}=200$ nm which allows the hybrid mode to confine in the low index medium and a large evanescent field resides in the upper cladding. The sensitivity of the waveguide is calculated by monitoring the change in the effective refractive index to the ambient refractive index. The maximum mode sensitivity of 0.7 is obtained. The spectral performance of the subwavelength grating (SWG) hybrid plasmonic waveguide (HPWG) based ring resonator design is studied which exhibits a sensitivity as high as 1000 nm/RIU which is higher than the previously reported values. We believe that the proposed waveguide scheme is an ideal candidate to produce highly sensitive lab-on-chip sensors.

On-chip nanophotonics and future challenges

AbstractOn-chip nanophotonic devices are a class of devices capable of controlling light on a chip to realize performance advantages over ordinary building blocks of integrated photonics. These ultra-fast and low-power nanoscale optoelectronic devices are aimed at high-performance computing, chemical, and biological sensing technologies, energy-efficient lighting, environmental monitoring and more. They are increasingly becoming an attractive building block in a variety of systems, which is attributed to their unique features of large evanescent field, compactness, and most importantly their ability to be configured according to the required application. This review summarizes recent advances of integrated nanophotonic devices and their demonstrated applications, including but not limited to, mid-infrared and overtone spectroscopy, all-optical processing on a chip, logic gates on a chip, and cryptography on a chip. The reviewed devices open up a new chapter in on-chip nanophotonics and enable the application of optical waveguides in a variety of optical systems, thus are aimed at accelerating the transition of nanophotonics from academia to the industry.

Simultaneous measurement of temperature and refractive index using microfiber knot resonators in Hi-Bi fiber loop mirrors

Based on the large evanescent field of microfibers, a novel fiber sensor using a microfiber knot resonator embedded in a Hi-Bi fiber loop mirror is proposed for simultaneous measurement of the temperature and refractive index of solutions. Due to the different responses of the microfiber knot resonator and fiber loop mirror to temperature and refractive index variations, the fiber sensor can measure temperature and refractive index simultaneously by utilizing the sensitivity matrix. The experimental measurements show that the sensitivities of the microfiber knot resonator and the fiber loop mirror to temperature are 5.6 pm/°C and −464.1 pm/°C, respectively. The sensitivity of the microfiber knot resonator to the refractive index of dilute solutions is 116 nm/RIU. The sensor has a higher sensitivity to refractive index variations than LPGs and has potential for use in the field of biochemistry.